1. Field of the Invention
This invention relates to an apparatus for measuring the tension load or the strain in a steel cable grouted in a borehole. More particularly, this invention relates to a low cost small size gauge to be attached to a steel cable. The gauge, which working principle is based on a resistance wire, is included in an electrical circuit for reading-out.
2. Description of the Prior Art
Steel cables, being either steel wire ropes or steel stranded cables, : have been used for a long time in reinforced and pre-stressed concrete in a variety of construction applications. More recently, the idea has emerged of using steel cables grouted in boreholes to reinforce rock masses and to ensure a better stability in situations such as road cuts, trenches, open pit and underground mines, or natural slopes nearby constructed areas. Typical steel cables often referred to as cable bolts when used in these latter applications, have a diameter ranging between 15 and 40 mm, a length varying between 5 to 50 m and are used in boreholes with a diameter between 45 and 90 mm. A grout which is usually a cement grout, a cement mortar or a chemical grout is pumped into the borehole after the cable has been pushed in.
Measuring tension in steel cables used either in a stand-alone application such as an elevator cable, a cable for a reel in deep-sea fishing, a cable in a mine hoist or a power line cable, or used in a combined application such as a reinforcement means in concrete structures is a rather common practice. Various apparatuses allowing for tension measurement are presently reported in patent literature, based on a variety of principles such as monitoring electrical parameters of a cable winding motor, inserting a piezo-element in the cable core heart, clamping a strain bridge to the cable, measuring the deflection of the cable under three point bending, bonding elements of different length and breaking strength on the cable measuring transverse vibrations induced by EM oscillators or finally using resistance strain gauge load cells. However, most of these systems cannot be used for the proposed application of measuring tension in a cable grouted in a small diameter borehole because their volume is too big which would impede subsequent proper injection of grout or require a too large and uneconomic borehole. Most of the systems already in existence are also very costly which does not encourage measurement of tension at a number of locations along the cable, whereas this practice is highly recommendable since it is well known that cables grouted in rock masses are tensioned rather locally because of the fractured nature of the rock. Also, in a natural rock mass, it is well recognized that loading of reinforcement cables is explained by one of the two following mechanisms: pure traction due to fracture opening or traction accompanied by shearing when some tangential movement is also taking place along the fractures. Most of the existing systems will give false readings in the event of shearing, whereas it is necessary that the principle on which the system would be based be independent of the mode of loading of the cable.
Finally, most of the existing systems were not designed for utilization on a grouted cable. It is therefore likely that movements of the cables due to a tension built-up will bring them in contact with the hardened grout and cause malfunctions.